1. Field of the Invention
The present invention relates to a reflection type liquid crystal display device and a manufacturing method thereof, and more particularly to a reflection type liquid crystal display device having a scattering reflector structure with high reflectance by a simple process, and a manufacturing method thereof.
2. Description of the Related Art
Recently in liquid crystal panels using an active matrix, reflection liquid crystal display devices, which can implement light weight, slimness and low power consumption, are attracting attention. A reflection type liquid crystal display device can decrease power consumption since light from the outside is taken inside the display panel and is reflected by a reflector installed at the rear face side, and backlight is unnecessary. Therefore the reflection type liquid crystal display device is useful as a display device for portable information terminals and portable telephones.
Light from the outside differs depending on the environment where the display device is used. Therefore it is desirable that the reflector installed in the display panel have a light scattering reflection structure which reflects light entering from a random direction to a random direction.
As such a reflection liquid crystal display device, a structure where pixel electrodes are formed on a bumpy shape film so that external light is irregularly reflected by the bumpy pixel electrodes has been proposed. For example, Japanese Patent Laid-Open No. H5-232465 and Japanese Patent Laid-Open No. H8-338993 proposed this structure. The reflection liquid crystal display device described in these publications uses photo-lithography processing using a mask pattern, or uses a combination of a polishing process and etching process in order to form undulation for pixel electrodes.
In these prior arts, high reflectance can be obtained by forming an arbitrary bump pattern at reflection electrodes.
But to control the shape of reflection electrodes using photo-lithography makes the process complicated. Since reflection characteristics change considerably if shape changes depending on the exposure conditions, the margin in the manufacturing process is small.
As a method of solving this problem, Japanese Patent Laid-Open No. H5-80327 discloses a method of simplifying the process using a thin film resin layer where the coefficient of thermal expansion is different from that of the reflection electrodes. With this method, however, undulation is formed on the surface of the pixel electrodes by forming a metal film by a heat sputtering method after organic film is formed. This method generates degassing from the organic film during the heating process in a vacuum, causing a change in the film quality of the reflection film or generating small undulation on the reflection film, which drops the reflection characteristics, therefore this process is not practical.
Japanese Patent Laid-Open 2000-193807 proposes a technology for forming fine undulation on organic films using fluorine-contained resin having a fluorine aliphatic ring structure for the main chain. This method, however, must use special resin, and requires a baking process at a high temperature of 350° C. Also, as the known example shows, this resin itself does not have photo-sensitivity, so if undulation is formed on pixel electrodes to be connected to a thin film transistor, resin must be coated separately to generate contact holes in the photo-lithography process, which makes the process complicated.
Also Japanese Patent Laid-Open No. H10-253977 states that undulation having variable distribution in the depth direction are formed using the intensity distribution of speckles which are generated when a coherent light is irradiated, so as to form a reflector having random bump distribution. This method, however, requires a special exposure system, and this exposure system is huge and has a high cost, which means that this method is not practical.
A plurality of undulation is formed at the surface of reflective electrodes of reflection type liquid-crystal devices in order to improve the optical scattering characteristic. Typically, in order to form the undulations at the surface of the reflective electrodes, the undulations are formed in the surface of the underlayer of the reflective electrodes. In this way, the undulations imitating the undulations of the underlayer are formed in the reflective electrodes.
An etching technique, of performing etching so as to produce a sine wave-shaped or triangular wave-shaped cross-sectional shape of the surface of an underlayer made of silicon oxide film (SiO2 film) is a known method of forming the undulations in the underlayer (see for example Laid-open Japanese Patent Application No. S56-156864 and Laid-open Japanese Patent Application No. S56-156865).
The following other techniques are known as methods of forming the undulations in the underlayer. First of all, a layer of photocured resin is formed as the underlayer. Next, the photocured resin layer is exposed using a photo-mask, in which a plurality of transparent regions are provided, for formation of the undulations. A photo-polymerization reaction is promoted in the exposed portions of the photocured resin layer, causing them to swell up relative to the unexposed portions, thereby forming raised portions. Next, further exposure of the photocured resin layer is performed using a photo-mask for contact hole formation. In this way, the photocured resin layer in the regions other than the regions where the contact holes are to be formed is cured, forming a plurality of the undulations in the surface. The contact holes are formed by subsequent development (see for example Laid-open Japanese Patent Application No. H11-153804).
In addition, the following technique is known as a method of forming the undulations in the underlayer. First of all, an underlayer is formed by coating photosensitive resin onto a substrate. Next, the photosensitive resin on the substrate is heated to partially different temperatures using a baking treatment device having a special hotplate. Solvent is evaporated from regions of the photosensitive resin that are heated to comparatively high temperature, decreasing its film thickness and resulting in the formation of surface undulations. The surface undulations of the photosensitive resin are maintained by performing baking treatment for a prescribed time. After this, an underlayer of undulated surface shape is formed through an exposure step and development step, using a photo-mask (see for example Laid-open Japanese Patent Application No. 2001-67017).
As a material used for an underlayer, a coating agent is also known containing a dye having a UV-absorbing capability (see for example Laid-open Japanese Patent Application No. 2001-348514). If such a coating agent is employed as the underlayer, UV is absorbed by the dye, so only the surface portions of the underlayer are cured by the UV irradiation and other portions of the surface of the underlayer are not cured. A difference between the surface portion and underlayer portion in the amount of shrinkage on curing is produced by heat, so the undulations are formed in the surface of the underlayer by subsequent heat treatment.
In addition, two Japanese Patent Applications (see Laid-open Japanese Patent Application No. 2002-296585, 2002-221716) filed by the present applicants proposes a technique for forming surface undulations of the underlayer by directing light of prescribed exposure energy onto the surface of the underlayer and subsequently performing heat treatment of the underlayer. These Japanese Applications are the corresponding ones of the co-pending parent U.S. patent application Ser. No. 10/051,709 assigned to the same assignee.
However, if SiO2 film is employed for the underlayer, a separate deposition process is required. Also, even if photosensitive resin is employed for the underlayer, either a new photo-mask is needed in the above technique, or a special manufacturing device or resin material is required.
Consequently, there is the problem that the manufacturing step of the liquid-crystal display device becomes complicated, increasing manufacturing costs.
A further problem is that the undulations are not reliably formed in the surface of the underlayer even by a method of irradiating the surface of the underlayer with light of a prescribed exposure energy and subsequently performing heat treatment [,as discussed in the above two Japanese Patent Applications].
As described above, various reflection type liquid crystal display devices where a scattering reflection electrode is used for a pixel electrode have been proposed, but in all cases, a scattering reflection electrode having sufficient reflectance cannot be formed with a simple manufacturing process. In order to form an optimum reflection electrode structure, it is necessary to control the average inclination angle of the undulation and the inclination angle distribution in an optimum range, but no manufacturing process which can control the average inclination and the inclination angle distribution to be an optimum reflection electrode structure with good repeatability has been proposed.
Also the inclination angle of the undulation of the reflector of a conventional reflection liquid crystal display device is selected such that maximum reflectance is obtained with respect to an incident light from a specific direction. A conventional reflector requires setting the inclination angle of the undulation to be 10°-20°, for example (Japanese Patent Laid-Open No. H11-259018), setting the inclination angle of the undulation of the reflector to be a uniform angle in a 5°-25° range (Japanese Patent Laid-Open No. H08-227071), setting the average inclination angle of the undulation of the reflector to be 30° or less (Japanese Patent Laid-Open No. S56-156865), with the heights of undulation in Gaussian distribution and the average inclination angle of the undulation at this time 10° (Tohru Koizumi and Tatsuo Uchida, Proceedings of the SID, Vol. 29, p. 157, 1988), and the surface of the reflector having a smooth bump face, and the average inclination angle of the undulation 4°-15° (Japanese Patent Laid-Open No. H6-175126).
In these prior arts however, no consideration was made concerning whether the reflectance becomes highest no matter from which direction the external lights enter the display panel. Therefore in the prior arts, no reflection type liquid crystal display device which becomes bright where external light is reflected at high reflectance under various environments have been proposed.
Also none of the prior arts proposed undulation shapes to make reflectance high, assuming a case when external lights enter the display panel of a notebook computer from all orientations at a certain direction and from a specific orientation at a direction which is different from that.
A reflector structure where a resist film is formed, exposed and developed with a predetermined mask pattern, then the cross-sectional structure of the resist film is smoothed by a baking process, so as to form a desired inclined face, has been proposed. However, in such a manufacturing process, an optimum pattern shape has not been proposed. A method of forming a undulation shape for reflection which has both directivity and scattering properties in a same pixel area has also not been proposed.
Also a reflection liquid crystal display device, which uses external lights, requires a light source to be used in a dark place. However, if a structure, where light from the light source is scattered and entered into the display panel side, is used, the displayed image is blurred by this scattering structure, which aggravates contrast.
The reflection type liquid crystal display device, which does not use backlight, can be slim, light and have low power consumption.
The reflection liquid crystal display device is roughly comprised of three layers, that is, a light shutter layer, a colored layer and a light reflection layer, but it is most important to obtain a bright display by utilizing ambient light efficiently. The light reflection layer of the above three layers has a particularly large influence not only on light utilization efficiency but also on viewing angle characteristics. Therefore optimizing the light reflection layer is most important to implement a bright reflection liquid crystal display device, and obtaining a bright light reflection layer has been considered.
Also a reflection type liquid crystal display device having a front light structure as an illumination system has been developed.
Also, by using a guest-host system where dichroic dye are mixed or a one polarizer system where one polarizer is used for the light shutter layer, a very bright display can be obtained in the former, and very high contrast can be obtained in the latter respectively in a bright state.
When the guest-host system where dichroic dye are mixed is used for the light shutter layer, considerable light leaks are generated if a diffuse reflector with high reflection efficiency is used, since the contrast of the guest-host liquid crystal is low in the dark state. In this state, the value of contrast of display characteristics is good, but the display does not look good visually.
Also if one diffuse reflector and one polarizer system are combined for a display, in this case, the display is good in the dark state, but brightness becomes insufficient in the bright state because of light absorption by the polarizer.
In the case of a reflection type liquid crystal display device having a front light structure as an illumination device, there are many interfaces between the liquid crystal substrate and the light guiding plate of the front light structure, therefore the light guided by the light guiding plate and the light directly entered from the outside is reflected at the interface without reaching the liquid crystal substrate. The light reflection which does not contribute to the liquid crystal display causes a drop in display quality, especially in contrast. Also, a reflection type liquid crystal display device primarily used for PDA normally has a touch panel on the surface. When the display device has a touch panel, there are also interface between the touch panel and the light guiding plate, therefore the above mentioned drop in contrast aggravates. Therefore it has been difficult to implement a reflection type liquid crystal panel having both a front light and a touch panel. As a countermeasure, a structure to decrease the reflection interfaces by integrating the light guiding plate of the front light structure and the touch panel has been considered, but the transparent conductive film used for the touch panel absorbs specific bands (blue and red, B, R) of the light being guided by the light guiding plate, and green becomes dominant on screen when combined with the light guiding plate.
Also, in the case of a prism type light guiding plate, a leak light component, which is directly emitted from the light guiding plate to the observer, is generated, which drops the contrast and makes particles which adhere to the surface of the prism more outstanding. This component is transmitted from the steep slope side of the prism face of the light guiding plate, which can be shielded to some extent, but the prism face of the light guiding plate is also a face where panel illumination light is generated, and it is difficult to implement both the shielding light to prevent leak light and panel illumination.
In this way, the reflection type liquid crystal display device can be slim, light weight and have low power consumption, but has serious problems due to a complicated manufacturing process and a narrowing of the manufacturing process margins, and it is difficult to improve the reflection characteristics.